There is a workaround to read mail content, but it is a bit user unfriendly. Might be useful to create automator action.
1. Copy base64 encoded text blocks from mail source and paste in text file with name mail.b64. The blocks of characters after
Content-Type: text/plain; charset="GB2312"
Content-Transfer-Encoding: base64
Should look like: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2. Use Terminal.app:
base64 -D -i <path_to>/mail.b64 -o mail.badcoding
3. In the same Terminal window:
iconv -f GB2312 -t utf8 <mail.badcoding
If charset you are having trouble with is ks_c_5601-1987, change last command to
iconv -f KS_C_5601-1987 -t utf8 <mail.badcoding